I feel like giving up

I (36m) have been seperated from my ex (29f) for 6 months. We have two kids together (f3.5) (m2). I had to move back in with my mum due to having no money as I paid for everything for previous 3 years'. She moved on after only two weeks but has kept the new boyfriend away from the kids. I go to her house or her mum's depending where they are sleeping as they often sleep at her mum's 3 nights a week. I have one night off a week where I don't see them to catch up on shopping and laundry. I have them both every Saturday and Sunday afternoon from 12-7.

Lately they have both stopped been excited to see me. Whenever there is something wrong or I tell them off, they both shout for their mum. They both cling to their grandma (my exes mum) more than anyone else. They get excited to see their mum. Im getting told by my little girl more and more over the past few weekends, that she wants her mum, or she wants to go home to her mum. I was reading her a bedtime story tonight and because I asked her to stop messing about as she kept getting in and out of bed, she had a meltdown screaming she wants her mum. I know I need to grow up and be more emotionally intelligent, but the constant rejection is really hurting me and I really don't want to hurt anymore. I love my kids with all my heart but I feel like in a permanent competition for their love. I always do the right thing for them, I entertain them, I don't just sit there on my phone, I play with them, I take them out to softplay, museums and parks. I'm getting really down, I'm tired of always doing the right thing and still feeling third best.

Do I take a break for a few days or do I just keep going as I am, I really don't know what to do for the best. I just tired of constantly subjecting myself to pain.

Any advice or insight would be greatly appreciated right now please.